Mineral Resources of theRuby Mountains Wilderness StudyArea, Madison County, Montana

U.S. GEOLOGICAL SURVEY BULLETIN 1724-A

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Chapter A

Mineral Resources of theRuby Mountains Wilderness StudyArea, Madison County, Montana

By R. G. TYSDAL, G. K. LEE,J. H. HASSEMER, and W. F. HANNAU.S. Geological Survey

S. W. SCHMAUCH U.S. Bureau of Mines

With a section on Talc

By R. B. BERGMontana Bureau of Mines and Geology

U.S. GEOLOGICAL SURVEY BULLETIN 1724

MINERAL RESOURCES OF WILDERNESS STUDY AREAS- SOUTHWESTERN MONTANA

DEPARTMENT OF THE INTERIOR

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987

For sale by theBooks and Open-File Reports SectionU.S. Geological SurveyFederal CenterBox 25425Denver, CO 80225

Library of Congress Cataloging in Publication Data

Mineral resources of the Ruby Mountains Wilderness Study Area, Madison County, Montana

(U.S. Geological Survey bulletin ; 1724-A)(Mineral resources ofwilderness study areas southwestern Montana; ch. A)

Bibliography: p. Supt. of Docs. No.: I 19.3:1724 1. Mines and mineral resources Montana Ruby Mountains

Wilderness. 2. Ruby Mountains Wilderness (Mont.) I. Tysdal, Russell G. II. Series: Geological Survey bulletin ; 1724-A.

III. Series: Mineral resouces of wilderness study areas southwesternMontana ; ch. A.

QE75.B9 no. 1724-A 557.3s 86-607948 [TN24.M9] [553'.09786'663]

STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

The Federal Land Policy and Management Act (Public Law 94-979, October 21, 1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral surveys on certain areas to determine the mineral values, if any, that may be present. Results must be made available to the public and be submitted to the President and the Congress. This report presents the results of a mineral survey of part of the Ruby Mountains Wilderness Study Area (MT-076-001), Madison County, Montana.

CONTENTSSummary 1 Introduction 3

Investigations by the U.S. Bureau of Mines 3 Investigations by the U.S. Geological Survey 3

Appraisal of identified resources 3Mining and mineral exploration history 3 Quality and quantity of identified resources 5 Mineral economics 5

Assessment of potential for undiscovered resources 5 Geology 5

Geologic setting 5 Description of rock units 8

Geochemistry 9Sampling and analytical methods 9 Results of study 9

Geophysics 10 Methods 10Discussion of anomalies 12

Mineral and energy resources 15 Iron 15 Talc 17Other mineral resources 20 Energy resources 20

References cited 20 Appendix 23

PLATE

1. Map showing mineral resource potential and geology of the Ruby Mountains study area In pocket

FIGURES

1. Map showing areas of mineral resource potential and prospects in the Ruby Moun­ tains study area 2

2. Index map showing location of wilderness study area in the Ruby Mountains, southwestern Montana 4

3. Complete Bouguer gravity anomaly map and generalized geology in the vicinity of the Ruby Mountains study area 16

4. Aeromagnetic anomaly map and generalized geology in the vicinity of the Ruby Mountains study area 18

TABLES

1. Mines and prospects in and adjacent to the Ruby Mountains study area 62. Analyses of stream-sediment samples, Ruby Mountains study area 103. Analyses of heavy-mineral concentrate samples, Ruby Mountains study area 124. Analyses of rock samples, Ruby Mountains study area 14

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS SOUTHWESTERN MONTANA

Mineral Resources of theRuby Mountains Wilderness Study Area,Madison County, Montana

By R. C. Tysdal, G. K. Lee, J. H. Hassemer, and\N. F. Hanna U.S. Geological Survey

S. W. Schmauch U.S. Bureau of Mines

With a section on Talc

By R. B. BergMontana Bureau of Mines and Geology

Summary

A mineral resource survey of the Ruby Mountains Wilderness Study Area (MT-076-001) was made in 1984- 85. Thirty-two million tons of subeconomic mineral re­ sources that average 21 percent iron occur in the Kelly iron deposit and constitute an identified resource adja­ cent to the southeasternmost edge of the study area. Moderate and high mineral resource potential for talc deposits occur within a large body of Archean (see geologic time chart in appendix) marble in the central part of the study area (fig. 1). The study area has a low mineral resource potential for uranium and for oil and gas sources. No phosphate deposits exist in the study area.

The Ruby Mountains Wilderness Study Area con­ tains 15,615 acres in the northern part of the Ruby Range, Madison County, Montana (figs. 1, 2). Elevation of the study area ranges from about 5,000 to 9,400 feet (ft). The mountains at the east and north margins of the study area rise abruptly from the alluvial fans that flank the mountain range, but on the west the topographic contrast is less abrupt. The area is accessible by roads extending south and southwest from Montana State Highway 287, east from Montana State Highway 41, and west from the Upper Ruby River road.

The Ruby Range lies in the foreland of the Rocky Mountains and is composed of a basement of Archean metamorphic rocks. Sedimentary rocks of limestone, do­ lomite, and minor sandstone and shale overlie the base­

ment in much of the study area and have an aggregate thickness of about 5,000 ft. Younger formations of con­ solidated sedimentary rocks, although at one time pres­ ent in the area, have largely been eroded.

Rocks of the Ruby Range were deformed during the Laramide orogeny. Folds, and associated faults with vertical displacements of as much as a mile, formed dur­ ing the course of the deformation and control erosion, giving rise to some of the present-day topographic con­ trasts within the range. The later development of distinct basins and ranges in southwestern Montana coincided with vertical displacements along faults that delimit the present-day margins of the Ruby Range. These basin- and-range faults separate the mountainous wilderness study area from downdropped basins of sedimentary rock that underlie the Beaverhead River Valley on the west and the Ruby River valley to the north and east.

Field and laboratory studies were conducted to as­ sess the mineral resource potential of the Ruby Moun­ tains Wilderness Study Area. Much of the area had been mapped previously, but reconnaissance traverses were made to examine the rocks for evidence of mineral de­ posits of various types, and detailed study of Archean marble units was undertaken because they may contain undiscovered talc deposits. A geochemical study in­ cluded spectrographic and chemical analyses of stream sediments, heavy-mineral concentrates, and rock sam­ ples collected by the U.S. Geological Survey (USGS); and examination, sampling, and analysis of rocks from mining claims, prospects, and mineral occurrences by the U.S. Bureau of Mines (USBM).

Ruby Mountains Wilderness Study Area A1

Area of identified iron resources

Geologic terrane having high resource potential For talc with certainty level C

Geologic terrane having moderate re­ source potential For talc with certainty levels B and C, as indicated

Prospect

APPROXIMATE BOUNDARY

VOF RUBY MOUNTAINS

WILDERNESS STUDY AREA (MT-076-001

U B Y

Figure 1. Map showing areas of mineral resource potential and prospects in the Ruby Mountains study area, Madison County, southwestern Montana. Extent of Ruby Range shown by dashed line.

A2 Mineral Resources of Wilderness Study Areas Southwestern Montana

Prospecting for metals has taken place in and adja­ cent to the study area, as is made evident by numerous prospect pits along the east flank of the range. Most of the pits likely were excavated in a search for base (cop­ per, lead, zinc) or precious (gold, silver) metals, but no deposits of this type were found. Identified resources of iron exist in the southeasternmost corner of the study area part of the Kelly iron deposit that lies mainly on private land immediately outside the boundary of the area (fig. 1) in the Archean metamorphic rocks that form the basement of the region. Moderate and high potential for talc occur in Archean dolomitic marble in the central part of the study area. The talc formed from siliceous fluids that reacted with the dolomitic marble, an event that most likely occurred during Precambrian time but after the 2,750 m.y. metamorphic event.

A low potential exists for energy resources within the entire wilderness study area. Uranium occurrences were not found in the area, and the geologic setting of the immediate area is not considered favorable for oil and gas resources. Phosphate-bearing sedimentary strata have been eroded from the range.

INTRODUCTION

The USGS and the USBM studied 15,615 acres of the Ruby Mountains Wilderness Study Area (MT-076- 001). The study of this acreage was requested by the U.S. Bureau of Land Management (BLM). In this report the area studied is called the "wilderness study area" or sim­ ply the "study area." The study area is located at the north end of the Ruby Range in Madison County, Mon­ tana (fig. 2, pi. 1). It is accessible by roads that extend south and southwest from Montana State Highway 287; east from Montana State Highway 41; and west from the Upper Ruby River road, which lies east of the wilderness study area. The elevation at the foot of the mountains is about 5,000 ft and reaches a maximum of 9,391 ft atop Ruby Peak, along the backbone of the range.

This report presents an evaluation of the mineral endowment (identified resources and mineral resource po­ tential) of the study area and is the product of several separate studies by the U.S. Bureau of Mines (USBM) and the U.S. Geological Survey (USGS). Identified re­ sources are classified according to the system of the USBM and the USGS (1980) which is shown in the ap­ pendix of this report. Mineral resource potential is the likelihood of occurrence of undiscovered concentrations of metals and nonmetals, of unappraised industrial rocks and minerals, and of undiscovered energy sources (coal, oil, gas, oil shale, and geothermal sources). It is classified according to the system of Goudarzi (1984) and is shown in the appendix of this report. The field work was carried out during the summers of 1984 and 1985.

Investigations by the U.S. Bureau of Mines

USBM personnel collected information about cur­ rent and past mining activities from county mining rec­ ords, pertinent literature, and from BLM mining records. A search was conducted in 1984 for all mines, prospects, mineralized zones, and claims. Properties were sampled and mapped where warranted. A total of 63 rock samples were collected, and all were analyzed for gold and silver; some also were analyzed for manganese, iron, copper, lead, zinc, or molybdenum; many were also analyzed by a semiquantitative spectrophotometric method for 43 ele­ ments. The complete results of the USBM work in this study area are included in Schmauch (1985).

Investigations by the U.S. Geological Survey

During the summers of 1984 and 1985, stream-sedi­ ment and heavy-mineral concentrate samples were col­ lected from streams that drain the wilderness study area and were analyzed chemically and spectrographically. Selected rock samples were analyzed chemically to aid in the interpretation of the stream-sediment and concen­ trate data. New gravity measurements were made in and adjacent to the wilderness study area to more precisely define gravity anomaly features previously mapped by Petkewich (1972). Under a contract with the USGS, R. B. Berg, Montana Bureau of Mines and Geology (MBMG), carried out a detailed study of those Archean (older than 2,500 m.y.) rocks believed to be possible hosts for talc deposits. In this report, the term "deposit," unmodified, carries no connotation of economic value. The mineral resource potential was classified by type of deposit, level of potential, and level of certainty.

Acknowledgments. The USGS and the USBM thank the many land owners in the area who granted us permission to cross their land to obtain data from the wil­ derness study area. The BLM staff at Dillon, Montana, provided data for the USBM investigations. John R. Benham, U.S. Bureau of Mines, participated in the USBM field studies. Professor R. M. Petkewich, Georgia State University, kindly provided the USGS with gravity data from his geologic work (Petkewich, 1972) in the ba­ sins marginal to the wilderness study area. That data aided in the geophysical interpretation.

APPRAISAL OF IDENTIFIED RESOURCES

By S. W. Schmauch U.S. Bureau of Mines

Mining and Mineral Exploration History

Mining claims and prospects were examined by the USBM (pi. 1 and table 1). Eight mining claims were lo-

Ruby Mountains Wilderness Study Area A3

112°30'

APPROXIMATE BOUNDARYOF RUBY MOUNTAINS

WILDERNESS STUDY AREA

Figure 2. Index map showing location of Ruby Mountains study area in the Ruby Range, Madison County, southwest­ ern Montana.

A4 Mineral Resources of Wilderness Study Areas Southwestern Montana

cated along the eastern flank of the Ruby Range between 1886 and 1894, mainly on iron or manganese stains. At Spring Canyon in the northwestern part of the study area, a claim (loc. 2P, pi. 1) was located for metals in 1923. A claim (loc. 3P, pi. 1) in the same Spring Canyon, and another near Ruby Peak (loc. IP, pi. 1), were claimed for talc in 1953 and 1956, respectively; the latter claim was refiled in 1976.

Ten claims were located in Taylor Canyon in 1897; they are adjacent to the wilderness study area. Two of these, the Copper Queen and Rattlesnake (both at loc. 10P, pi. 1), were patented in 1901 but remain unde­ veloped. The Kelly iron deposit was discovered in 1932 and held by assessment, work until 1963. During the late 1950's, several bulldozer trenches and at least two drill holes were used to explore the iron-formation. No current mining claims exist within the study area.

Quality and Quantity of Identified Resources

Only a small part of the Kelly iron deposit lies with­ in the wilderness study area. The principal area of the deposit lies outside the study area and is estimated to con­ tain 32 million tons that average 21 percent iron; the de­ posit is considered to be a subeconomic resource.

Mineral Economics

Identified resources at the Kelly iron deposit could be developed and mined concurrently with other similar iron deposits in the general region. A centralized or possi­ bly portable processing facility could concentrate the iron ore prior to rail shipment; rail transportation and talc proc­ essing facilities are located on both sides of the Ruby Range. An increase in the value of iron ore and (or) the identification of a larger resource is needed for develop­ ment. Open-pit mining at the Kelly mine would likely disturb an area of less than 1 sq mi. The small portion of this deposit that lies inside the study area is too small and too remote to extract profitably at this time.

The term "marble" as used in this report, refers to limestone and (or) dolomite that has been recrystallized by metamorphism into medium- or coarse-grained meta- limestone or metadolomite. It lacks most of the desirable physical characteristics associated with use as a building stone, and no resources were identified. Minor amounts of talc occur in the dolomitic horizons of the Archean marble in the study area. No identified resources of talc occur in the study area.

ASSESSMENT OF POTENTIAL FOR UNDISCOVERED RESOURCES

By R. G. Tysdal, G. K. Lee, J. H. Hassemer, andW. F. HannaU.S. Geological Survey

Geology

Geologic Setting

The northern part of the Ruby Range, in which the wilderness study area lies, consists of a basement of Ar­ chean metamorphic rocks overlain by Paleozoic, Mesozo- ic, and Cenozoic sedimentary strata (see appendix for geologic time scale). The basement rocks were originally metamorphosed to a high metamorphic grade about 2,750 m.y. (James and Hedge, 1980) and underwent a second and milder thermal event about 1,600 m.y. (Giletti, 1966). The metamorphic rocks of the range have been referred to part of the Cherry Creek Group because of the abundance of rocks of probable sedimentary origin, including marble units (several hundred feet thick), quartzite, and iron-formation; however, correlation of these highly deformed and recrystallized rocks is difficult (James and Hedge, 1980), thus lithologic terms are used in this report.

The Paleozoic strata aggregate about 5,000 ft thick and are composed largely of limestone and dolomite typi­ cal of the Rocky Mountain foreland in southwestern Mon­ tana. Mesozoic strata are represented only by rocks of the Late Cretaceous Beaverhead Group. Other Mesozoic strata of marine and nonmarine origin probably were de­ posited and subsequently eroded before deposition of the Beaverhead (Tysdal, 1976b).

Igneous rocks intruded the sedimentary strata after Late Cretaceous-Tertiary (Laramide) deformation. The in­ trusive rocks are chiefly sills and small-diameter (a few tens of feet across) pipelike bodies of basalt; andesitic and rhyolitic pipelike intrusions are present locally. A vol­ canic unit of andesitic tuff breccia and agglomerate is present in one place along the west side of the range (pi.1).

During the Laramide orogeny, the mountain range was broken into blocks defined by steeply dipping faults that trend north and northwest. The faults record as much as 7,000 ft of vertical displacement. Sedimentary strata on the uplifted sides of faults were deformed into anti­ clines, and strata on the downthrown sides were deformed into synclines. The folds plunge moderately to the north­ west (Tysdal, 1976a; 1981).

Ruby Mountains Wilderness Study Area A5

Tab

le 1

. M

ines

and

pro

spec

ts i

n an

d ad

jace

nt

to t

he

Rub

y M

ou

nta

ins

stud

y ar

ea

[Pro

perti

es w

ithin

the

stud

y ar

ea a

re u

nder

scor

ed]

n 2 2 § sr 3

No.

(plate 1)

IP

Property Name

Unknown _

'(9Q

S SI

Summary

Workings

Massive limestone strikes northwest and

dips

One

pit.

Sample data

One

grab sample contained no significant values.

2P 3P 4P 5P 6P 7P

Ellen group

Robinson

Canyon area

Laurin Canyon

area

Porier Canyon

area

lenses of

calcareous siltstone.

Competent, massive- to medium-bedded gray

limestone strikes N. 55° W.

, dips 40° NE.

An area of secondary limonite-hematite

stained limestone is

5

ft thick, 20 ft

long,

and

20 ft

deep.

Blocky, iron-stained limestone with dark gray

chert

nodules ha

s minor interbeds of red

to

green sh

ale.

Th

e property wa

s located fo

r ta

lc in

19

52.

Red-brown shale with scattered calcite veins

and

veinlets 0.

2 to 0.

7 ft

thick in

contact

with massive gray limestone.

The

rocks

strike N. 55

° W.

an

d dip 30

° to 35

° NE.

Massive limonite-stained cherty limestone is

locally brecciated.

Pink-red quartzite interbedded .wi

th buff-colored

limestone strikes northwest an

d dips 50

° to

70°

NE.

Incompetent, massive, buff-colored limestone with

hairline fracture fillings of

manganese.

The

extent of

th

e manganese could not

be determined

beyond pi

t limits.

One pit

and

a 22-ft-deep

inclined shaft.

Several bulldozer cu

ts and

two

prominent 100-ft-long

by 50-ft-high bench cuts.

Three small pits and

a caved adit estimated to

be le

ss than 50 ft

lo

ng.

A 34-ft-long tr

ench

, and

a pi

t.

One

13-ft-long adit an

d a

60-ft-long trench.

Two

shallow pits.

Two

select and

three chip samples from th

e iron-stained le

ns contained no significant

values.

Seven chip or

grab samples from limestone and

shale contained no ta

lc.

One chip an

d four grab samples contained no

significant values.

Three samples contained no

significant va

lues

,

One

chip an

d tw

o grab samples contained no

significant values.

One chip an

d two

grab samples contained

0.05

%, 0.15%, an

d 0.13% manganese,

respectively.

8P 9P

Unknown

' (1

4, 6, 5)

Morning St

ar

Decomposed tan-brown dolomite with hairline

One

shallow pi

t.

fracture fillings of

li

raon

ite

and manganese.

The 2.3-ft thick le

ns could no

t be traced

beyond pit

limits.

Limestone strikes N.

10°

W. an

d dips 30°-35° NE.

None

. A

10-ft

by 30-ft outcrop ha

s numerous closely-

spaced joints filled with limonite.

One chip and

one grab sample each

contained 0.22% manganese.

One grab sample contained no significant values.

Copper Queen-

Rattlesnake Lode

A 1-

to 2-ft-thick shear zone strikes N.

70

W.

an

d dips 26

° to 36° SW fo

r about 100

ft along

a dolomite-quartzite unconformity.

The shear

zone

is

comprised of

go

uge,

calcite veins,

limonite, quartz lenses, an

d at on

e location,

malachite stains.

Two adits, 26 and 40

ft

long,

and

six

shallow

pits

.

A to

tal

of nine chip samples were taken across the

shear zone.

Three from the

40-ft ad

it contained

0.06%, 0.

07%,

an

d 0.04% copper.

One other chip

sample contained 4.2% zinc.

UP

Lance No

s.

1 and 2

12P

Kelly Ir

on

Deposit

A 20-ft-thick band of

quartz-magnetite iron

formation strikes N. 45° W.

and

dips southwest

and

is partly exposed for

700

ft in decomposed

schist and gneiss.

This is

pa

rt of

the

outermost band described in th

e Kelly Iron-

deposit.

Precambrian gneiss, sc

hist

, and

bands of

quartz-

magnetite iron-formation ar

e folded in

to an

east-southeast plunging syncline.

A 1,000-ft

by 4,000-ft upward-faulted trough section

contains the

most highly contorted an

d thickest

(100 to 300 ft)

iron-formation ba

nds.

Results

from two

drill holes in

th

is area averaged

34%

iron to

a

depth of 500

ft (J

ames

and

Wier,

1972

).

One shallow pit.

Seven closely spaced

bulldozer trenches from

100

ft to

40

0 ft

long

and

from 2

to 10

ft

de

ep,

and

one

pit.

A random ch

ip and a

grab sample assayed 32.2% and

9.5% iron,

respectively.

Twenty-two random chip or

grab samples ranged

from 9.3% to

37.5% iron,

but

averaged 22.1%

iron.

The trough section has

over one-half of

the

total outcrop exposures, an

d covers over

0.5

million sq

ft.

If iron-formation bands in

these exposures are

consistent to a depth of

500

ft,

32 million tons of

subeconomic iron

resources averaging at

le

ast

21%

iron exists.

Several properties listed as

"unknown" could no

t be

identified by

claim notices or

through published reports.

The

numbers in parentheses following "unknown"

represent locations by

section, township, and range.

Underlined names indicate properties inside th

e study ar

ea.

(From Schmauch, 1985)

73 or O 3_ £T

Concurrent with faulting and differential displace­ ment across the faults, the mountains were eroded, and some of the material was preserved locally as the coarse elastics of the Beaverhead Group. Later during deposition of the Beaverhead, quartzite and siltite clasts (pebbles and cobbles) from the Proterozoic Belt Supergroup flooded in from the west and are preserved in the youngest strata of the Beaverhead within the Ruby Range. The final stage of the orogeny is recorded in the placement of discrete blocks of Archean and Paleozoic strata along the west, north, and to a lesser extent, the east flank of the northern third of the Ruby Range. These blocks were interpreted as gravity slide blocks by Tysdal (1970, 1976a), although Schmidt and Garihan (1983, map p. 274) interpreted a few of those along the western side of the mountains as thrust slices. The northern part of the range is flanked by Tertiary normal faults, which separate the mountains from the basins.

Description of Rock Units

Rock units within or immediately adjacent to the wilderness study area are described below, with letter symbols as shown on plate 1. Descriptions of the Archean (older than 2,500 m.y.) rock units are adapted from James and Wier (1972) and Karasevich (1980, 1981); descrip­ tions of the younger units are from Tysdal (1976a, b). No relative age is implied by the order of the Archean rock units.

Archean quartzofeldspathic gneiss (Agn) Most widespread unit is well foliated, medium- to coarse­ grained gneiss. The light-colored variety is chiefly a quartz-orthoclase-plagioclase rock. The medium- to dark- colored varieties are composed of quartz-plagioclase- biotite or quartz-plagioclase-biotote-hornblende assem­ blages. Common accessory minerals include garnet, zir­ con, and apatite.

Archean marble (Am) White, tan, pale-green, and brown dolomitic and calcitic marble that is fine to coarse grained. Layering is well defined by thin seams of quartz; by accessory minerals such as tremolite, pyroxene, biotite, muscovite, or graphite; or by differences in the iron con­ tent of the marble. Forms units as much as 3,000 ft thick.

Archean amphibolite (Aa) Dark-green to black hornblendic gneiss that commonly forms lenses and pods a few tens of feet thick. Minor or accessory minerals in­ clude clinopyroxene, biotite, apatite, and zircon. Near the iron-formation adjacent to the southeast corner of the wil­ derness study area, gneissic amphibolite contains abundant pyroxene.

Archean ultramafic rock (Ac) Dark-green to black, fine- to coarse-grained metapyroxenite and metaperidotite in which the pyroxene and olivine are extensively altered. Occurs only in area of iron-formation.

Archean iron-formation (Ai) Dark-colored, fine- to medium-grained, well-layered rock composed of magne­ tite, quartz, and minor garnet and clinopyroxene. Beds as thick as several tens of feet. Present only adjacent to the southeast corner of the wilderness study area.

Undivided sedimentary rocks of Cambrian age ( u) Chiefly limestone and dolomite, with lesser shale and sandstone. In descending order: the Red Lion Forma­ tion is mainly light-brown dolomite, with domal and col­ umnar stromatolites in the uppermost part; thickness 115- 170 ft. The Pilgrim Dolomite is brown, thin- to medium- bedded dolomite, except for crossbedded quartz sandstone in the uppermost few feet of the formation. The Park Shale is olive-green clay shale; 150-200 ft thick. The Meagher Limestone is light-gray, thin-bedded limestone, except where it has been dolomitized and is now pale yellowish brown; it ranges from about 550 to 750 ft thick. The Wolsey Shale is olive-green finely micaceous clay shale and siltstone; it is 60-100 ft thick. The Flathead Sandstone consists of light-colored, fine- to coarse-grained quartz sandstone; about 60 ft thick; unconformably over­ lies the Archean metamorphic rocks.

Undivided sedimentary rocks of Mississippian and Devonian age (MDu) Chiefly limestone and dolomite. The upper part is the Devonian and Mississippian Three Forks Formation, which consists of an upper member of grayish-orange siltstone; a middle member of olive-green clay shale; and a lower member of light-gray, thin- to thick-bedded limestone. Total thickness of the Three Forks is about 150 ft. The underlying Jefferson Dolomite is yellowish-brown, medium- to thick-bedded, dense, fetid dolomite, which is about 250 ft thick.

Undivided sedimentary rocks of Mississippian age (Mu) Mainly limestone. The Kibbey(?) Formation, at the top of the sequence, is brick-red, thin- to medium-bedded dolomite, silty dolomite, and siltstone. The top of the Kibbey(?) has been eroded, but the remaining rocks range from about 20 to 140 ft thick. The Mission Canyon Lime­ stone is light-gray-weathering, medium- and thick-bedded cliff-forming limestone, which contains evaporite-solution breccia zones in the upper part and chert stringers in the lower part. The formation is about 1,400 ft thick. The upper half of the underlying Lodgepole Limestone is light- gray, medium- to thick-bedded limestone that is abun­ dantly fossiliferous; the lower half of the formation is yellowish-brown thin-bedded argillaceous limestone. Total thickness of the Lodgepole is about 600 ft.

Beaverhead Group of Cretaceous age (Kb) Reddish-orange conglomerate composed of well-rounded cobbles of quartzite and siltite clasts eroded from the Proterozoic Belt Supergroup; cobbles and boulders of limestone eroded from Paleozoic formations; and clasts derived from metamorphic rocks of the range. The group is preserved in isolated outcrops on the west side of the range. Total thickness as much as 150 ft.

A8 Mineral Resources of Wilderness Study Areas Southwestern Montana

Volcanic rocks of Tertiary age (Tv) Dark-colored andesitic tuff breccia, lapilli tuff, and agglomerate that is crudely bedded and contains well-rounded andesite clasts. The unit is preserved only in one place along the west side of the wilderness study area.

Intrusive rocks of Tertiary age (Tib, Tia, Tir) Sills and pipelike intrusive bodies of basalt, and less commonly of andesite and rhyolite, chiefly along the west flank of the range.

Bozeman Group of Tertiary age (Tb) Conglomer­ ate. Along the western side of the range, the upper part of the unit is composed mainly of quartzite clasts; along the east flank it is composed of clasts eroded from Paleo­ zoic formations. The lower part of the group is olive to gray tuffaceous mudstone, siltstone, and sandstone. Total thickness is several hundred feet.

Sediments of Quaternary age (Qu) Alluvium, allu­ vial fans, colluvium, landslide deposits, and glacial de­ posits.

Geochemistry

Sampling and Analytical Methods

Stream-sediment samples were collected from most active first order (unbranched) drainages in the wilderness study area, as well as from all second order and larger streams. At each site a composite sample of fine detritus was taken from several places within the stream and later air dried for sieving and analysis. Heavy-mineral concen trates of stream sediments were collected near the stream- sediment site, but were derived from coarser material thought to represent a relatively high-energy depositional environment. The heavy-mineral concentrates were ob­ tained by panning; the samples were sent to the laboratory for drying and analysis.

Rock samples were collected to (1) identify and evaluate places where obvious mineralized or altered rock existed within the wilderness study area, and (2) deter­ mine background abundances of elements to help evaluate the stream-sediment samples. The samples usually were taken as representative composites of rock chips from out­ crops, but the most mineralized or altered rocks were col­ lected preferentially in some cases.

All samples were analyzed semiquantitatively for 31 elements using a six-step, direct-current arc, optical- emission, spectrographic method (Grimes and Marran- zino, 1968). Stream-sediment samples were sieved through an 80-mesh (177 micron) screen, and a split of the portion passing through the sieve was analyzed. A split of each crushed and finely ground rock sample, and a small split of each heavy-mineral concentrate sample, were analyzed by this method. Atomic absorption analyses for gold were made on the remainder of each heavy-

mineral concentrate sample, using the method of Thompson and others (1968). Rocks were analyzed for antimony, arsenic, bismuth, cadmium, and zinc using the atomic-absorption method of Viets and others (1984), in which the readily soluble portion of these elements is dis­ solved and analyzed; and for gold using the method of Thompson and others (1968).

Results of Study

Stream-sediment samples locally show values of chromium or nickel that are anomalous for the limestone terrane across which the streams flow. Heavy-mineral concentrate samples from these and a few other sites con­ tain elevated values of manganese, nickel, chromium, and locally, cobalt. These elements form a suite common in mafic or ultramafic rocks, and most of the sediments probably were derived in part from basalt intrusives pres­ ent locally within the limestone terrane. At Spring Canyon in the northwestern part of the wilderness study area, the likely sources of this suite of elements in the streams are the tiny pipelike basalt bodies that intruded the Beaverhead Group. Analysis of the basalt (table 4, sample no. 84MTzl32) shows elements of this suite. In the lime­ stone terrane of Laurin Canyon, in the east-central part of the wilderness study area, the elevated values in con­ centrate samples are from streams that have small basaltic intrusive bodies in their upper reaches. At the mouth of the limestone-bearing Porier Canyon, concentrate sample 14S (table 3) has elevated values of chromium and manga­ nese for which no immediate source is known; the values probably reflect an unmapped basaltic intrusive body in the drainage.

At Dry Creek, on the western edge of the wilder­ ness study area, concentrate samples IS and 26S (table 3 and pi. 1) contain values of manganese that are slightly elevated for a limestone terrane. No mafic intrusive bodies are known in the immediate area of the sample sites. However, the streams of this area cross amphibolite- bearing Archean metamorphic rocks within 1-3 mi of the sample sites; the values are not anomalous for Archean rocks. At the southern edge of the wilderness study area, manganese, nickel, and chrome in sample 18S (table 3 and pi. 1) are not considered anomalous for the crystalline rocks of the area. The values likely were derived from the several sill-like amphibolite bodies just upstream from the sample site.

A few concentrate samples from the western flank of the mountain range show some very low values for gold. The only known possible source of the element is conglomeratic strata of the Cretaceous Beaverhead Group. This formation is in part composed of clastic materials eroded from the Proterozoic Belt Supergroup, and the conglomeratic strata are known to contain traces of gold throughout the outcrop area in southwestern Montana. The

Ruby Mountains Wilderness Study Area A9

Table 2. Semiquantitative spectrographic analyses of

[N. M. Conklin, L. A. Bradley, and M. J. Malcolm, analysts, Branch of Analytical Chemistry. Analyses in ppm except where shown in

Location Sample LatitudeNumber Number deg-min-sec

IS RM001AS 45-19-472S RM002AS 45-21-323S RM003AS 45-21-454S RM004AS 45-22-265S RM005AS 45-22-39

6S RM006AS 45-22-077S RM007AS 45-21-318S RMD08AS 45-21-029S RM009AS 45-18-08

10S RM010AS 45-17-35

11S RM001GS 45-19-4212S RM002GS 45-19-4913S RM003GS 45-19-5014S RM004GS 45-19-2415S RM005GS 45-18-42

16S RM006GS 45-18-2817S RM007GS 45-19-3918S RM008GS 45-17-4619S RM001LS 45-19-4920S RM002LS 45-19-45

21S RM003LS 45-18-3322S RM004LS 45-18-3923S RM005LS 45-18-4024S RM006LS 45-18-5225S RM001PS 45-19-52

26S RM002PS 45-20-0927S RM003PS 45-20-5428S RM004PS 45-21-3229S RM005PS 45-21-3830S RM006PS 45-22-45

31S RM007PS 45-17-3232S RM008PS 45-17-16

Longitudedeg-min-sec

112-15-55112-14-54112-15-39112-15-45112-14-42

112-12-00112-11-38112-11-28112-09-23112-09-32

112-11-51112-11-45112-11-21112-10-17112-09-57

112-16-49112-17-29112-13-25112-12-51112-12-53

112-11-05112-12-12112-12-11112-11-25112-15-53

112-16-18112-16-03112-16-18112-14-46112-14-39

112-09-44112-09-01

gold-bearing samples are from streams thatareas of the Beaverhead andBeaverhead has been eroded.

Fe %(0.05)

1.51.51.511.5

0.7.7

113

1.51.51.511.5

1.5.7

51.57

32331.5

1.51.51.51.51.5

1.53

Mg %(0.02)

71.521.52

1.51.51.573

1.51.531.51.5

32575

10.7331.5

531.51.52

1.53

drain outcropareas from which the

Ca % Ti %(0.05) (0

7 037

105

151515

73

2535

10

715

371.5

310

735

53

1053

77

.002)

.15

.15

.15

.15.15

.15

.15

.15

.15

.3

.3

.3

.15

.15

.15

.15

.15

.3

.15

.3

.2

.15

.15

.15

.2

.2

.15

.2

.2

.15

.2

.2

Mn(10)

3,000700700500700

300300300500

1,500

500500

1,0001,500

300

300200

1,000300

1,500

700700

1,500700

1,000

3,000500700700700

7001,500

Ag(0.5)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

As(700)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Au(15)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

B Ba(10) (30)

30 30070 30030 30015 30070 300

30 15070 20030 15015 20030 200

50 30070 20020 15030 20070 200

30 300L 200L 30070 15030 150

30 30070 20020 200

100 30070 300

30 30070 30070 30070 30070 200

30 300L 150

Be(1)

1LLLL

LLLLL

11LLL

1L

1LL

1.5111.51

LLLLL

LL

Geophysics

MethodsA few rock samples show elevated values of arse­

nic, antimony, and zinc, elements commonly associated with precious metals (gold and silver). No measurable amount of precious metals were found in rocks of the area, however. Samples 4R, 5R, and 6R (table 4) reflect hydrothermal activity in and adjacent to silicified and (or) limonitic fracture zones in the vicinity of the Copper Queen-Rattlesnake prospect in Taylor Canyon (loc. 10P, pi. 1). USBM data for this prospect (table 1) show minor copper and some zinc, but no gold or silver. One stream- concentrate sample (loc. 9S, pi. 1) from this general area showed a very low gold value and probably reflects the same hydrothermal activity. Sample 8R (table 4), from a small fracture zone in the Ruby Peak area, contains some zinc but no measurable amount of precious metals.

Gravity data within and in the vicinity of the wilder­ ness study area consist of 160 measurements made by J. H. Hassemer using a LaCoste and Romberg geodetic gravimeter G-2 1 . The principal facts for these gravity measurements are available in a report by Hassemer and Kaufmann (1986). Sixteen of the measurements were made within the boundary of the wilderness study area. All measurements are referenced to a gravity value at a base station in Dillon, Montana (Kaufmann and others, 1983), incorporated into the International Gravity Standardization Net 1971.

'Use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey

A10 Mineral Resources of Wilderness Study Areas Southwestern Montana

stream-sediment samples, Ruby Mountains study area

percent. Limit of determination shown in parentheses. L, less than such limit; >, greater than value shown. Data reported in 6-step classification,

Bi(10)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Cd(30)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Co(5)

77

1057

5775

10

77777

7L

201020

77

15157

107777

715

Cr(10)

50501503050

30303030

100

5030705030

3030

30070150

701001507070

7050305050

70150

Cu(5)

3030502030

1515101530

3030302015

1515701530

3030303030

3030303030

3030

La(30)

7030LL30

LLLLL

30LLLL

LL150L150

70L5030L

LLLLL

LL

Mo(5)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Nb(20)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Ni(5)

2030501520

1530151530

2030201515

1515

1503070

1515302030

3030302030

3070

Pb(10)

3030151520

1010101515

1515151515

15L

151515

1515151515

2030201515

1510

Sb Sc(100) (5)

L 15L 10L 15L 7L

LLL

7

777

L ' 5L 7

LLLLL

LLLLL

LLLLL

LLLLL

LL

77757

75

157

30

101015157

77777

715

Sn(10)

LLLLL

LLLLL

L20LLL

LLLL10

LLLL15

1515LLL

L15

Sr(100)

150200300150300

300300300150150

150150150150300

300300300150L

150300L150300

150150300200150

200150

V(10)

7070703050

3050303070

3070503050

5030

10070

150

701001007070

7070707050

7070

W(50)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Y(10)

3030203030

1515151515

3020151515

1510701550

3015201520

2015202020

2020

Zn(200)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Zr(10)

200150150150150

507070

15070

2001507070

100

15070

30070

150

20070

150150150

100150150300150

10050

Th(200)

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LLLLL

LL

Observed gravity was computed using calibration coefficients established by laboratory bench and mountain loop calibrations, and corrections for earth tide and instru­ mental drift were made using a computer program de­ veloped by M. W. Webring, R. R. Wahl, and G. I. Even- den of the USGS. Bouguer gravity anomalies with atten­ dant terrain and earth curvature corrections, assuming a reduction density of 2.67 g/cm3 (grams per cubic centi­ meter), were computed relative to theoretical gravity val­ ues derived for Geodetic Reference System 1967 (Wool- lard, 1979). Equations used in this gravity reduction are summarized in Cordell and others (1982) and are im­ plemented in a computer program of R. H. Godson, USGS. Terrain corrections to a distance of 175 ft (Ham­ mer, 1939) from each measurement point (station) were visually estimated during the field work. Terrain correc­ tions beyond 175 ft, and extending to a distance of 103 mi from each station, were computed using a modification

of the method of Plouff (1977) and incorporating U.S. Department of Defense terrain data digitized at a 15- second grid interval. Also included in the data base are 14 stations of Kaufmann and others (1983) and 12 stations from the U.S. Defense Mapping Agency file. For pur­ poses of editing and display, gravity-station locations were projected to x-y coordinates, and Bouguer gravity- anomaly values corresponding to these points were gridded using a minimum curvature algorithm (Webring, 1981). The gridded anomaly data were subsequently con­ toured using an algorithm for splining under tension (God­ son and Webring, 1982) and were machine plotted. The complete Bouguer anomaly map shown in figure 3 repre­ sents only part of the larger area surveyed.

The aeromagnetic-anomaly map (fig. 4) is an en­ largement of part of a map of southwestern Montana and east-central Idaho (USGS, 1975) compiled from data ac­ quired at a barometric elevation of 12,000 ft along east-

Ruby Mountains Wilderness Study Area A11

Table 3. Semiquantitative spectrographic and chemical analyses

[R. T. Hopkins, spectrographic analyst; M. A. Pokorny and T. A. Roemer, chemical analysts, Branch of Exploration Research. Analyses in determination; >, greater than value shown. Data reported in 6-step classification, at midpoints 10, 7, 5, 3, 2, 1.5, 1, and so forth]

Location Number

IS2S3S4S5S

6S7S8S9S

10S

US12S13S14S15S

16S17S18S19S20 S

21S22S23S24S25S

26S27S28S29S30 S

31S32S

Sample Number

RM001ACRM002ACRM003ACRMOO 4ACRM005AC

RMD06ACRMOO 7 ACRMOO SACRM009ACRM010AC

RM001GCRMOO 2GCRM003GCRMOO 4GCRM005GC

RMOO 6GCRM007GCRM008GCRM001LCRMOO 2LC

RM003LCRMOO 4LCRM005LCRM006LCRM001PC

RM002PCRM003PCRM004PCRM005PCRM006PC

RM007PCRM008PC

Latitude deg-min-sec

45-19-4745-21-3245-21-4545-22-2645-22-39

45-22-0745-21-3145-21-0245-18-0845-17-35

45-19-4245-19-4945-19-5045-19-2445-18-42

45-18-2845-19-3945-17-4645-19-4945-19-45

45-18-3345-18-3945-18-4045-18-5245-19-52

45-20-0945-20-5445-21-3245-21-3845-22-45

45-17-3245-17-16

Longitude deg-min-sec

112-15-55112-14-54112-15-39112-15-45112-14-42

112-12-00112-11-38112-11-27112-09-23112-09-32

112-11-51112-11-45112-11-21112-10-17112-09-57

112-16-49112-17-29112-13-25112-12-51112-12-53

112-11-05112-12-12112-12-11112-11-25112-15-53

112-16-18112-16-03112-16-18112-14-46112-14-39

112-09-44112-09-01

Fe % (0.1)

7751.53

1.53237

102577

37

1520

7

35777

521.552

315

Mg % (0.05)

53515

1.51.51.557

35573

22237

1.53352

5511.55

55

Ca % (0.1)

107

203020

3050302015

32010

77

2015

33

15

75357

1515201020

205

Ti % (0.005)

10.5

.5

.15

.2

.15

.15

.15

.15

.3

.5

.15

.3

.7

.7

.3

.722

.3

.2

.51

.5

.5

.2

.15

.15

.5

.15

.151.5

Mn (10)

5,0001,0002,000

300500

300300500

1,5005,000

3,0001,0002,0007,0002,000

7002,0005,0007,0001,500

7003,0003,0003,0005,000

5,000500500700500

2,0007,000

Ag (1)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

As (500)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Au (20)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

B (20)

10070LN

200

7070703050

2070502070

10070N3070

50705050

150

10010050

100100

70N

Ba (50)

L300300

N100

L200

L7050

LL50L

150

50150

LNL

70150

LL

100

1505070

150L

200L

Be (2)

LLLN2

LNLLN

NNLLL

LNLNL

LLNLL

L2NNL

LN

Bi (20)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Cd (50)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

west flight lines spaced approximately 2 mi apart. The 1:250,000-scale map was optically enlarged to approxi­ mately 1:110,000 and anomalies were digitized by contour tracing. The digital data were then projected, gridded, and recontoured as described above.

Physical properties of four Archean rock types marble, biotite schist, talc schist, and iron-formation were measured to determine whether their magnetizations or densities are sufficiently anomalous to serve as sources of magnetic and gravity anomalies. The marble is non­ magnetic but dense (2.84 g/cm3). The biotite schist is very weakly magnetic (negligible remanent magnetization; in­ duced magnetization of about 10" 1 A/m (amps per meter) and very dense (3.65 g/cm3 , although on the basis of other work nearby, this value may be much higher than aver­ age)). The talc schist is essentially nonmagnetic and somewhat dense (2.76 g/cm3). The iron-formation is ex­ tremely magnetic (remanent magnetization of 102 A/m; induced magnetization of 1.15X102 A/m) and very dense

(2.98 g/cm3). Thus, all of the Archean rock types sampled are expectedly dense and are candidate sources of gravity highs. However, only iron-formation is strongly magnetic and a candidate source of high-amplitude magnetic anomalies.

Discussion of Anomalies

The gravity anomaly map (fig. 3) is dominated by a broad high bordered by gradients that are conformable with margins of mapped rocks of the northern part of the Ruby Range (Tysdal, 1976a; Karasevich, 1981; Pet- kewich, 1972; Burfiend, 1967; and Kaufmann and Hanna, 1982). Within the wilderness study area, the high appears as a 10-mgal circular enclosure that tapers southwestward to a small elliptical high in the vicinity of Beatch Canyon.

Steep gradients on the northeastern and northwest­ ern flanks of the circular high, in the center of the wilder-

A12 Mineral Resources of Wilderness Study Areas Southwestern Montana

of heavy-mineral concentrate samples, Ruby Mountains study area

ppm except where shown in percent; c, chemical. Limit of determination shown in parentheses. N, not detected; L, less than limit of

Co (10)

152020

NN

NNNN

15

20N

102015

NL

203015

L20502020

10NNLN

N50

Cr (20)

100200

1,0003070

20303070

200

30050

100200100

70100500500150

70200200200200

10030507050

100700

Cu (10)

103050

L30

1010101020

15101530

L

10L

202050

1515201550

2020102020

1020

La (50)

7050

LLL

LNNNN

NLL500

70

N50

1,000300N

505070

L50

5050

L50

L

NN

Mo (10)

NLNNN

NNNNN

NNNNN

NNNNN

NNNNN

NLNNN

NN

Nb (50)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Ni (10)

2070

1001020

2030201050

7020507050

3020

1507050

5070

1007050

7050152015

20150

Pb (20)

2070203030

30LL

20N

LLN

2020

203050

L20

50LNL

20

3050202020

L20

Sb (200)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Sc (10)

15L

10NN

NNNN

20

20NL

2015

N153030

L

L30703050

10NNNN

L20

Sn (20)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Sr (200)

N300

NNN

SOU500500

NN

NLNNN

500NNNN

NNNNN

LNLLL

NN

V (20)

150200200

5070

30707050

150

1505070

100150

70100300300

70

70100150100100

707050

15050

70200

W (100)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Y (20)

70LLNL

NNNN30

50N507070

L50

500150

20

L70

10015050

50NLLL

L50

Zn (500)

NNNNN

NNNNN

NNNNN

NN,NNN

NNNNN

NNNNN

NL

Zr (20)

5007050

300200

20505050

500

707050

1,500500

501,5002,000

500100

100150100300100

707070

150100

70150

Th (200*)

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NNNNN

NN

Au c(0.05)

LNNNN

NLN

0.05L

NNNNN

.1NLLN

NNNNL

LNN.1L

LL

ness study area, follow the outline of mapped rocks. The gradients are generated by the contrast between the low- density alluvium and underlying basin-fill deposits of the intermontane basins and the adjacent higher density folded Phanerozoic (Paleozoic and Mesozoic) rocks and Archean metamorphic rocks of the mountain range. The steepness of the gradients suggests that these contacts are within 15 degrees of vertical and thus are probably fault contacts. The steep gradient on the southwestern flank of the high is generated by low-density basin fill deposits in lateral contact with Archean metamorphic rocks.

The small elliptical gravity high in the Beatch Can­ yon area is associated with the Kelly iron deposit. This iron-formation consists primarily of magnetite-rich layers, thinner quartz-rich layers, and ferromagnesian silicates, indicating that the deposit is anomalously dense and mag­ netic. The absence of well-defined gradients along con­ tacts of Phanerozoic sedimentary rocks and Archean metamorphic rocks in the southern part of the area shown

on figure 3 is unexpected, although data are somewhat sparse in this region. This absence of gradients implies that the folded Phanerozoic rocks are underlain by Arche­ an metamorphic rocks at shallow depth throughout the area of the figure.

The aeromagnetic anomaly map (fig. 4) is domi­ nated by a 200-gamma (nT, or nanoTesla) circular high centered over the area of the Kelly iron deposit, where a flight line fortuitously crossed this terrane. Gradients flanking this anomaly extend southwestward to the south­ west corner of the map area of figure 4, where part of another circular high is present; this high is possibly associated with concealed iron-formation in metamorphic rocks that are more than 5 mi south of the wilderness study area. The oval 50-gamma low that broadly covers the northern extremity of the Ruby Range, including most of the wilderness study area, is an expression of the rela­ tively nonmagnetic character of the Phanerozoic and Ar­ chean rocks; the low is not an expression of reversed rem-

Ruby Mountains Wilderness Study Area A13

Tabl

e 4.

S

em

iqu

an

tita

tive

sp

ect

rog

rap

hic

and

ch

em

ica

l an

alys

es o

f ro

ck s

ampl

es,

Rub

y M

ou

nta

ins

study

area

[N.

M.

Con

klin

, L.

A

. B

radl

ey,

and

M.

J.

Mal

colm

, sp

ectr

ogra

phic

ana

lyst

s, B

ranc

h of

Ana

lytic

al C

hem

istr

y; P

. H

. B

rigg

s an

d J.

G.

Cro

ck,

chem

ical

an

alys

ts,

Bra

nch

of A

naly

tical

Che

mis

try.

A

naly

ses

in p

pm e

xcep

t w

here

sho

wn

in p

erce

nt;

c,

chem

ical

. L

imit

of d

eter

min

atio

n sh

own

in p

aren

thes

es;

L,

less

th

an

such

lim

it;

>,

grea

ter

than

va

lue

show

n;

-, no

t de

term

ined

. D

ata

repo

rted

in

6-

step

cl

assi

fica

tion,

at

m

idpo

ints

10

, 7,

5,

3,

2,

1.

5,

1, a

nd s

o fo

rth]

Location

Number

1R 2R 3R 4R 5R 6R 7R 8R 9R10R

10R

10R

Sample

Number

84MTzl30

84MTzl31

84MTzl32

84MTz234

84MTz235

84MTz236

NR17

NR40

RM001AR

RM009PRA

RM009PRB

RM009PRC

Latitude

deg-min-sec

45-19-57

45-20-37

45-21-35

45-17-43

45-17-43

45-17-37

45-19-22

45-18-22

45-19-51

45-17-19

45-17-19

45-17-19

Longitude

deg-min-sec

112-15-49

112-

16-1

1112-15-03

112-10-19

112-10-07

112-10-00

112-14-00

112-13-20

112-16-04

112-09-25

112-09-25

112-09-25

Rock

Fe %

Type

(0.05)

Red

siltstone

Lapilli tuff

Basalt

Limonitic marble

Hematitic sandstone

Oxidized sandstone

Limonitic marble

Limonitic quartzite

Quartz pegmatite

Rusty quartz pegmatite

Iron-f ormation

Banded gneiss

1.5

5 7 0.7

5 1.7

20.7 .1

20 3

Mg %

(0.02)

0.7

1.5

3 7.7 .1

57 1.

51.

5 .07

2 3

Ca %

(0.05)

15 3 7151.

5

2 15 0.7

2.0

5

1.5

5

Ti %

(0.002)

0.3 .3 .5 .01

.15

.05

L .15

L .007

.07

.15

Mn

(10) 20

01,

500

1,500

3,000

150

703,000

1,50

070

015

0

>5,000

1,50

0

Ag

(0.5

)

L L L L L L L L L L L L

As

(700)

L L L L L L L L L L L L

Au

(15)'

L L L L L L L L L L L

B (10) 50L L L 70

15L L L L L L

Ba

(30)

1,500

2,000

2,000 70

300 70 70

150 20

150

500

5,000

Be

(1) L 1.5

1 L L L L L L L L L

Bi

(10) L L L L L L L L L L L L

Cd

(30) L L L L L L L L L L L L

Location

Number

1R 2R 3R 4R 5R 6R 7R 8R 9R

10R

10R

10R

Co

(5) L 15 15 5 10 5 L

20L L 10 20

Cr

(10) 70 30

200 10

100 L L

150 L L 70

500

Cu

(5)

715 150 5 L L 70

150

7,000 20 100

70

La

(30) 70 70 50 L L L L L L L L L

Mo

(5) L L L L L L L L L L L L

Nb

(20) L L L L L L L

20L L L L

Ni

(5) 7 770 20 30 207

70L L 5

150

Pb

(10) L

20 15 L L L L L L L L L

Sb

(100)

L L L L L L L L L L L L

Sc

(5) 7

15 30L 7 5 L

50L L 7

15

Sn

(10) L L L L L L L - L L L L

Sr

(100)

300

700

1,500 L

300

150 L L L L L 150

V (10) 30 70

300 10 30 15 L 70 L L 20 70

W

(50)

L L L L L L L L L L L L

Y (10) 30 50 30 L 30 L L

300 L L L 10

Zn

(200)

L L L L L L L L L L L300

Zr

(10)

300

300

200 70 150 70 L 100 L L 20 70

Th

(200

)

L L L L L L L L L L L L

As

c(0.

5) L L L189

23 19 L 7 L L L

Bi

c(2) L L L L L L L L L L L L

Cd

c(O.l)

0.2 L 0.1

0.2

0.2

0.1

0.2

0.6 L L 0.5

0.1

Sb

c(2) L L L20 29 39 11 L L L L L

Zn

c(2) 8

83 66 6 8

225

47L L 6

52

Au

c(O.

l)

L L L L L L L L - - - -

nant magnetization in metamorphic rocks. A weak high, forming a northwest-trending lineament on the southwest flank of this 50-gamma (nT) low, expresses a probable slight increase in magnetization of Archean rocks insuffi­ cient in volume to cause a corresponding gravity ridge. Likewise, local mapped occurrences of andesite, basalt, iron-formation, and serpentinized ultramafic rocks are in­ sufficient in size to be detected by the widely spaced high- altitude flight lines.

Mineral and Energy Resources

Iron

The model (Kelly iron deposit). Banded iron- formation is a minor but widespread rock unit in the Ar­ chean metamorphic terrane of southwestern Montana. Iron-formation of the Kelly iron deposit exists within a tiny area of the southeastern part of the wilderness study area. The principal area of the Kelly deposit lies im­ mediately adjacent to the southeast corner of the wilder­ ness study area (in sec. 26, T. 6 S., R. 5 W., pi. 1). A thin (averaging about 30 ft thick), oval-shaped belt of iron-formation nearly surrounds the principal part of the deposit; a small fraction of this thin belt occurs within the wilderness study area.

The Kelly deposit is an oxide facies iron-formation consisting chiefly of magnetite and quartz, with subordi­ nate pyroxene and garnet. The iron-formation is dark gray to black, fine to medium grained, and streaky to well layered. It readily weathers into a reddish-brown soil. Iron-formation is associated with rocks that probably are of sedimentary origin dolomite and elastics and is in­ ferred to have been a chemical sediment deposited in a shallow-water environment. The sequence was meta­ morphosed about 2,750 m.y. (James and Hedge, 1980) and underwent a second regional metamorphism about 1,600 m.y. (Giletti, 1966).

The Kelly deposit as mapped by James and Wier (1972) contains outcropping iron-formation and inferred iron-formation between outcrops as determined from trenches, a ground magnetometer survey, and from tracing the distribution of reddish-brown soil typical of weathered iron-formation. The layers of iron-formation are generally only a few feet thick, although in the principal area of the Kelly deposit layers are several tens of feet thick. The layers probably were of more uniform thickness when originally deposited, but they underwent pinching and swelling during deformation and associated metamorph­ ism. Karasevich and others (1981) indicated that the metamorphic rocks in and near the Kelly deposit have undergone as many as four phases of folding.

Chemical analyses of two drill core samples from the Kelly deposit were reported by Bay ley and James (1973, p. 954). The average composition (weight percent) for the oxides in the two cores was reported by James

(1981); the most abundant oxides are as follows: Fe2O3 , 32.49; FeO, 19.05; SiO2 , 39.79; MgO, 2.43; and CaO, 1.88.

The Kelly iron deposit did not produce a prominent geochemical signature in stream-sediment and heavy-min­ eral concentrate samples. As previously noted, the Kelly deposit is associated with a high-amplitude aeromagnetic high.

Resource potential. The Kelly iron deposit is an identified resource that has the following characteristics: (1) outcrop of iron-formation, and (2) a prominent aeromagnetic and gravity high. In addition, the deposit was trenched and drilled by the Finlen and Sheridan Com­ pany in the 1950's.

Most layers of iron-formation of the Kelly deposit lie immediately adjacent to the southeast corner of the wilderness study area. At least one layer of iron-formation of the deposit extends into a tiny area of the southeast- ernmost part of the wilderness study area, near Taylor Canyon (northeasternmost part of sec. 26, T. 6 S., R. 5 W., and southeastern corner sec. 24, T. 6 S. R. 5 W., pi. 1). This layer and its associated metamorphic rock are separated from the remainder of the wilderness study area by the South Fork fault (Kephart fault of Karasevich, 1980, 1981; and Karasevich and others, 1981) on the west, and on the north by an unnamed east-trending fault in Taylor Canyon.

Characteristics of the remainder of the area of out­ cropping Archean metamorphic rock within the wilderness study area were compared to those of the terrane of the Kelly deposit. With the exception of the tiny area de­ scribed above, the metamorphic terrane within the bound­ aries of the wilderness study area lacks outcrops of iron- formation, shows readily apparent source rock other than iron-formation for values of elements in stream-sediment and heavy-mineral concentrate samples, and is not charac­ terized by gravity and magnetic anomalies normally as­ sociated with massive occurrences of iron-formation. This part of the wilderness study area is assigned a low poten­ tial for iron-formation, with a certainty level of C (avail­ able data gives a good indication of the level of mineral resource potential). The classification system used here is that of Goudarzi (1984), (see inside front cover) and is shown on plate 1.

In the part of the wilderness study area where metamorphic terrane is concealed by younger sedimentary rocks, criterion 1 above cannot be evaluated because metamorphic rocks are not exposed to examination. How­ ever, with regard to criterion 2, and in consideration of the flight height and spacing of the aeromagnetic survey and station spacing of the gravity survey, we may infer that no iron-formation deposit even a tenth the size or a tenth the magnetite content of the Kelly iron deposit occurs within the metamorphic rocks veneered by Meso- zoic and Paleozoic rocks. The concealed area of metamor-

Ruby Mountains Wilderness Study Area A15

Figure 3 (above and facing page). Complete Bouguer gravity anomaly map and generalized geology in the vicinity of the Ruby Mountains study area.

A16 Mineral Resources of Wilderness Study Areas Southwestern Montana

EXPLANATION

Gravity contours Contour interval, 2 milligals; hachures indicate depres-

190 sion; dots show location of measure­ ment stations; H, gravity high

Contact

. NormaLfault, concealed D, downthrownside; U, upthrown side

Area of identified iron resources

Quaternary and Tertiary basin deposits

Mesozoic and Paleozoic sedimentaryrocks

Archean metamorphic rocks

phic rock is judged to have a low resource potential for iron-formation, with a certainty level of B (available in­ formation suggests the level of mineral resource poten­ tial). Iron-formation in deposits very much smaller than that of the Kelly may exist beneath the sedimentary cover and not be detected by the existing aeromagnetic data; similarly, they could exist beneath the surface in areas of exposed Archean rocks.

Talc

By R. B. BergMontana Bureau of Mines and Geology

This section on mineral resource potential for talc is a summary of a contract study (Berg, 1987) carried out as a part of the USGS's evaluation of the mineral resource potential of the wilderness study area.

The model (talc in Archean marble). Exposures of Archean marble were examined for talc; and soil, scree, and talus were searched for chips of talc and fragments of talc-bearing marble. Talc fragments can be readily rec­ ognized in the scree below inaccessible steep cliffs. In several places where talc chips were unusually abundant, the concentration of talc could be traced uphill to a source vein or pod. Although talc fragments are typically dark colored because of a coating of iron and manganese min­ erals, they are easily recognizable by their smooth surface and ease with which they can be scratched with the finger­ nail. The amount of discoloration of the talc fragments gives a crude indication of the distance that the talc has traveled from its source; in general, talc fragments less than several feet from their source are pale green and free of discoloration (Berg, 1987).

The concentration of talc occurrences within an area gives an indication of the likelihood for the occurrence of a significant deposit. As an example, in the Greenhorn Range (12 mi southeast of the wilderness study area), talc

occurrences are much more abundant in the southern half of the range where the only talc mine in this range (inac­ tive as of 1985) is situated. Berg (1979, sheets 2 and 3) showed 43 talc occurrences in the range, most of which are concentrations of talc chips in the soil. Although dolomitic marble is exposed throughout the Greenhorn Range, 33 of the 43 talc occurrences are in the southern part of the range. The density of talc occurrences in the southern part of the range is 2.2/mi2 of marble, whereas in the northern part of the range it is 0.7/mi2 . No density concentrations were calculated for the entire wilderness study area of the Ruby Range, but in the parts of the wilderness study area rated as having high potential for talc, the density concentrations for talc occurrences are much greater than 2.2/mi2 (see pi. 1).

Host rock composition is the only recognized con­ trol of talc within the boundaries of the wilderness study area. Talc occurrences are limited to Archean marble, of which three varieties have been recognized in the wilder­ ness study area. Type I marble, estimated to constitute 20 percent of the marble of the wilderness study area, is calcitic and contains no talc. Type II marble, estimated to make up 50 percent of the marble of the wilderness study area, consists of calcite, dolomite, and minor acces­ sory minerals, and does not commonly contain talc. Type III marble, estimated to make up 30 percent of the marble of the wilderness study area, is dolomitic and contains most of the talc of the wilderness study area. (A detailed description of the characteristics of the three types of Ar­ chean marble is presented in Berg, 1987).

No evidence of structural control of talc occurrences was found in the wilderness study area. Quartz layers and boudins (lenses) up to about 1 ft thick in the general area of Ruby Peak have been deformed locally into folds rang­ ing from open to very tight (isoclinal), with the axial sur­ faces of the isoclinal folds subparallel to compositional layering of the enclosing marble. Folds are generally con­ fined to individual compositional layers about 10 to 15 ft thick within the marble. Talc concentrations were not found to be controlled by these folds. No concentrations of talc were found near the high-angle faults that jux­ tapose Paleozoic sedimentary rocks against the Archean metamorphic rocks.

Talc is unknown in the dolomitic formations of Pa­ leozoic age in southwestern Montana, even though the dolomite would be an ideal host for the mineralizing solu­ tions; this suggests that the talc formed during the Precam- brian, after the metamorphic event of 2,700 m.y. Garihan (1973) suggested that talc formation can be correlated with the l,600-m.y.-old thermal event that affected the metamorphic rocks of southwestern Montana.

An estimated 80 percent of the observed talc occur­ rences are within Type III marble, where the talc forms veinlets, small pods, and conformable layers. Veinlets range up to 1/2 in. thick and are scattered throughout the

Ruby Mountains Wilderness Study Area A17

Figure 4 (above and facing page). Aeromagnetic anomaly map and generalized geology in the vicinity of the Ruby Mountains study area.

A18 Mineral Resources of Wilderness Study Areas Southwestern Montana

100'

EXPLANATION

Magnetic contours Contour interval, 10 gammas; hachures indicate depres­ sion; H, area of magnetic high and value in gammas; L, area of magnetic low and value in gammas

Flight path of airborne survey

Contact

Normal fault, concealed D, downthrown side; U, upthrown side

Area of identified iron resources

Quaternary and Tertiary basin deposits

Mesozoic and Paleozoic sedimentaryrocks

Archean metamorphic rocks

marble. Blebs or pods less than several inches long are also found scattered throughout Type III marble, but are not found in any significant concentration except in the talcose layers described below.

Talcose pods or layers concordant with foliation of Type III marble contain most of the talc found in the wilderness study area. More than 50 percent, and perhaps as much as 70 percent, of all of the talc seen is in these layers. Eight of these layers were found and all have sev­ eral features in common. With the exception of one oc­ currence (locality 2T, pi. 1, described below) they are traceable for no more than 6 ft along strike, typically pinching out at both ends. Thickness ranges from 1 to 25 in., and estimated talc content ranges from 5 to 90 percent. Most of these layers contain calcite and less abundant dolomite. A large pod of talcose marble is ex­ posed in the cliffs on the south side of the ridge in the SW'ASE'ASW'A sec. 8, T. 6 S., R. 5 W. (pi. 1). This pod is 13 by 13 by 6 ft and is estimated to contain 5 to 10 percent talc in dolomitic marble.

At locality IT (pi. 1) abundant pieces of talc 2 to 8 in. long indicate a concentration of talc in the marble of a prospect pit. Within a distance of several tens of feet to the south, many small talc pods (less than 4 in. long) occur within an exposure of Type III marble. A conformable layer (about 3 ft thick) consisting of a mix­ ture of talc, chlorite, and quartz is exposed approximately 6 ft along strike to the northwest from the prospect. This layer is covered at the southeast end and pinches out at the northwest end. Talc float is abundant for several tens of feet on the slopes to the northwest. A rock consisting of a mixture of chalcedony, dolomite, and talc, some with the texture of weathered bone, also was found in float approximately 150 ft northwest of the concentration of talc. On the basis of the abundance of talc float in this

area, it is possible that there are abundant talc pods or layers within the dolomitic marble.

At locality 2T (pi. 1) a poorly exposed conformable talcose layer occurs; it is 6 ft thick, extends for about 75 ft along strike, and is in Type III marble. Barren mar­ ble of Type III is exposed to the northeast and soil covers the layer to the northwest. Talc was not found in the Type III marble exposed either above or below the talc- bearing layer. On the basis of loose blocks of talc, this 6x75 ft area either is a single body of talc or, more likely, an area in which the dolomitic marble has been largely replaced by talc.

Resource potential. The mineral resource potential for talc in the various parts of the wilderness study area ranges from low to high. High potential with level C cer­ tainty is applied to three areas: (1) surrounding Ruby Peak (part of sec. 16, T. 6 S., R. 5 W.), (2) west of Ruby Peak (SE^NE^ sec. 17, T. 6 S., R. 5 W.), and (3) a small area in the S'/2 of sec. 8, T. 6 S., R. 5 W. Because of sparse vegetation in the three areas, they could be searched effectively for talc. On the basis of the aforementioned comparison to the Greenhorn Range, an area with similar bedrock, the concentration of talc oc­ currences in the three areas of the wilderness study area must be considered anomalous. A higher level of certainty would require additional data. The remainder of the large area underlain by marble in the vicinity of Ruby Peak is rated to have moderate resource potential with a level C certainty. The area is underlain by marble, in part dolomitic, and scattered occurrences of talc are in the float. Bedrock is generally well exposed over much of this area.

The two isolated areas underlain by marble in the NE'/4 sec. 7 and NW'/4 sec. 8, T. 6 S., R. 5 W. are rated to have moderate resource potential for talc, with level B certainty; they are poorly exposed. The ridge un­ derlain by marble at the western boundary of the wilder­ ness study area, in SW'/4 sec. 12, T. 6 S., R. 6 W., is rated to have moderate potential with level C certainty. The marble is well exposed and, although talc float was not found within the wilderness study area, a small amount of talc float was found along this same layer 250 m west of the wilderness study area. Marble is exposed at the southeastern corner of the wilderness study area in the SE'/4 sec. 23 and SW'/4 sec. 24, T. 6 S., R. 5 W. (pi. 1). Exposures of bed rock in this area are good. Because the area is underlain by marble, some of which is dolomitic, it is considered to have moderate talc poten­ tial; the level of certainty is judged to be C. The remain­ der of the area of outcropping metamorphic rocks in the wilderness study area is considered to have a low talc resource potential because of a lack of Archean marble.

The resource potential assigned to the individual areas of the exposed metamorphic rocks is based on geologic parameters that are useful only for rocks at or

Ruby Mountains Wilderness Study Area A19

near the surface. Neither these parameters, nor geochemi- cal and geophysical parameters, were considered useful to assess the likelihood of the occurrence of concealed talc resources. Hence, the favorafrility and certainty rat­ ings apply only to those metamorphic rocks at or near the surface; the mineral resource potential for talc at depth in these areas may be lower or higher than those deter­ mined for exposed metamorphic rocks.

The mineral resource potential for talc in metamor­ phic rocks concealed beneath Mesozoic and Paleozoic strata is unknown, and the certainty is rated A: available information is not adequate for determination of the level of mineral resource potential. The existence and extent of marble beneath the cover of Mesozoic and Paleozoic strata are unknown, and the geologic parameters used to determine the mineral resource potential for talc are not useful in areas that are covered.

A mining company was negotiating in 1984 for mineral leases on private land near the wilderness study area and has staked claims on talc discoveries in parts of sections 19, 20, 29, and 30, T. 6 S., R. 5 W., and sections 11 and 12, T. 6 S., R. 6 W. (pi. 1) just to the south of the wilderness study area (Howard Harlan, Cyprus Industrial Minerals Company, written commun., October 5, 1984).

Other Mineral Resources

No phosphate deposits are known to occur in the wilderness study area. The Permian Phosphoria Forma­ tion, the sedimentary host rock for phosphate in south­ western Montana, has been eroded from the area. A possi­ bility exists that the Ruby Range could be underlain by a thrust fault, and a remote possibility exists that Phos­ phoria strata may occur beneath such a thrust fault. The mineral resource potential for phosphate in the study area is rated as low, with a certainty level of C.

Gold occurs in Archean rocks in nearby mountain ranges but no deposits are known in Archean rocks of the wilderness study area. No measurable amount of gold was found in the Archean rocks analyzed. One stream concentrate sample (9S, table 3) yielded a very low gold value; the sample is from the general area of a prospect (loc. 10P, pi. 1) in sheared, hydrothermally altered Arche­ an rocks. No measurable concentration of silver was found in the rocks of this general area. The mineral re­ source potential for gold and silver in Archean metamor­ phic rocks is rated as low, with a certainty level of C: available information gives a good indication of the level of mineral resource potential.

Gold, silver, and base metals (copper, lead, and zinc) occur in ranges to the east, north, and northwest of the wilderness study area, and apparently are related to Laramide-age granitic intrusive rocks. No such intru­ sive rocks are exposed within or near the study area and

no indications of concealed granitic intrusives were found during field studies or detected by geophysical studies. The mineral resource potential for gold, silver, and base metals associated with intrusive rocks is rated as low, with a certainty level of C: available information gives a good indication of the level of mineral resource potential.

Energy Resources

The mineral resource potential for uranium in the wilderness study area was evaluated by the U.S. Depart­ ment of Energy during the National Uranium Resource Evaluation (NURE) program. The area was not classified as favorable for uranium deposits (Wodzicki and Krason, 1981). As part of this program, Broxton (1979) reported no anomalous concentrations of uranium in stream-water or stream-sediment samples taken from the wilderness study area and the immediately adjacent land. The poten­ tial for uranium resources is rated as low, with a certainty level of C.

The oil and gas potential on Federal Wilderness Lands in Montana was evaluated by Perry and others (1983). To facilitate the evaluation, they subdivided the lands into geologically related appraisal areas. The apprai­ sal area within which the wilderness study area lies was assigned a low potential for oil and gas resources (Perry and others, 1983, p. G15). They noted that the appraisal area contains a broad distribution of Archean metamorphic rocks and Tertiary volcanic rocks at the surface, and lacks known source beds for hydrocarbons. Recent studies of the structural development of the foreland of southwestern Montana indicate that some areas of Archean metamorphic basement rocks have been displaced by thrust faults. Rec­ ognition of these thrust faults leads to a possibility that the wilderness study area could be underlain by a thrust fault, and if so, potentially hydrocarbon-bearing sedimen­ tary strata may exist beneath such a thrust. Seismic data or drilling is needed to determine if this possibility is real. The low potential of the wilderness study area is assigned a certainty level of B.

No geothermal resources are known to exist within or immediately adjacent to the wilderness study area.

REFERENCES CITED

Bayley, R. W., and James, H. L., 1973, Precambrian iron- formations of the United States: Economic Geology, v. 68, p. 934-959.

Berg, R. B., 1979, Talc and chlorite deposits in Montana: Mon­ tana Bureau of Mines and Geology Memoir 45, 66 p.

1987, Potential for talc deposits within the BLM Wilder­ ness Study Area of the northern part of the Ruby Range, Madison County, Montana: U.S. Geological Survey Open- File Report 87-001.

A20 Mineral Resources of Wilderness Study Areas Southwestern Montana

1987, Potential for talc deposits within the BLM Wilder­ ness Study Area of the northern part of the Ruby Mountains, Madison County, Montana: U.S. Geological Survey Open- File Report 87-001.

Broxton, D. E., 1979, Uranium hydrogeochemical and stream sediment reconnaissance of the Dillon NTMS quadrangle, Montana/Idaho, including concentrations of forty-three addi­ tional elements: U.S. Department of Energy Open File Re­ port GJBX-38, 228 p. (Los Alamos Scientific Laboratory Report LA-7347-MS).

Burfiend, W. J., 1967, A gravity investigation of the Tobacco Root Mountains, Jefferson Basin, Boulder batholith, and areas of southwestern Montana: Bloomington, Indiana Uni­ versity, Ph. D. dissertation, 90 p.

Cordell, L. E., Keller, G. R., and Hildenbrand, T. G., 1982, Bouguer gravity map of the Rio Grande Rift: U.S. Geologi­ cal Survey Geophysical Investigations Map GP-949, scale 1:1,000,000.

Garihan, J. M., 1973, Origin and controlling factors of the talc deposits of steatite grade in the central Ruby Range, south­ western Montana: Geological Society of America, North­ eastern Section Meeting, Abstracts with Programs, v. 5, no. 2, p. 164.

Giletti, B. J., 1966, Isotopic ages from southwestern Montana: Journal of Geophysical Research, v. 71, p. 4029-4036.

Godson, R. H., and Webring, M. W., 1982, CONTOUR: A modification of G.I. Evenden's general purpose contouring program: U.S. Geological Survey Open-File Report 82-797, 32 p.

Goudarzi, G. H., 1984, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey Open-File Report 84-787, 30 p.

Grimes, D. J., and Marranzino, A. P., 1968, Direct-current arc and alternating-current spark emission spectrographic field methods for the semiquantitative analysis of geologic mate­ rials: U.S. Geological Survey Circular 591, 6 p.

Hammer, Sigmund, 1939, Terrain corrections for gravimeter sta­ tions: Geophysics, v. 4, p. 184 194.

Hassemer, J. H., and Kaufmann, H. E., 1986, Principal facts for the gravity stations in the Dillon I°x2° quadrangle, Montana and Idaho: National Technical Information Service PB86-197407/AS,6p.

James, H. L., 1981, Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana: U.S. Geological Survey Professional Paper 1187, 16 p.

James, H. L, and Hedge, C. E., 1980, Age of the basement rocks of southwest Montana: Geological Society of America Bulletin, v. 91, p. 11-15.

James, H. L., and Wier, K. L., 1972, Geologic map of the Kelly iron deposit, Sec. 25., T. 6 S., R. 5 W., Madison County, Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-349, scale 1:800.

Karasevich, L. P., 1980, Structure of the pre-Beltian metamor- phic rocks of the northern Ruby Range, southwestern Mon­ tana: Pennsylvania State University, MS thesis, 172 p.

1981, Geologic map of the northern Ruby Range, Madi­ son County, Montana: Montana Bureau of Mines and Geol­ ogy Geologic Map 25, scale 1:24,000.

Karasevich, L. P., Garihan, J. M., Dahl, P. S., and Okuma, A. F., 1981, Summary of Precambrian metamorphic and structural history, Ruby Range, southwest Montana: Mon­ tana Geological Society Field Conference and Symposium Guidebook to Southwest Montana, p. 225-237.

Kaufmann, H. E., and Hanna, W. F., 1982, Slides showing preliminary mosaic aeromagnetic and complete Bouguer gravity anomaly maps of the Dillon I°x2° quadrangle, Montana and Idaho: U.S. Geological Survey Open-File Re­ port 82-604, 2 35 mm slides, 6 pages text.

Kaufmann, H. E., Sorenson, S. B., and O'Neill, K. J., 1983, Principal facts and complete Bouguer gravity anomaly map for the Dillon I°x2° quadrangle, Montana and Idaho: U.S. Geological Survey Open-File Report 83-51, 75 p.

Perry, W. J., Jr., Rice, D. D., and Maughan, E. K., 1983, Petroleum potential of wilderness lands in Montana: U.S. Geological Survey Circular 902-G, 23 p.

Petkewich, R. W., 1972, Tertiary geology and paleontology of the Beaverhead east area, southwestern Montana: Missoula, University of Montana, Ph.D. dissertation, 343 p.

Plouff, Donald, 1977, Preliminary documentation for a FOR­ TRAN program to compute gravity terrain corrections based on topography digitized on a geographic grid: U.S. Geologi­ cal Survey Open-File Report 77-535, 45 p.

Schmauch, S. W., 1985, Mineral resources of the Ruby Moun­ tains Study Area Madison County, Montana: U.S. Bureau of Mines Open File Report 63-85, 10 p.

Schmidt, C. J., and Garihan, J. M., 1983, Laramide tectonic development of the Rocky Mountain foreland of southwest­ ern Montana, in Lowell, J. D., and Gries, Robbie, eds., Rocky Mountain Foreland Basins and Uplifts: Denver, Rocky Mountain Association of Geologists, p. 271-294.

Thompson, C. E., Nakagawa, H. M.,and Van Sickle, G. H., 1968, Rapid analysis for gold in geologic materials, in Geological Survey Research: U.S. Geological Survey Pro­ fessional Paper 600-B, p. B130-B132.

Tysdal, R. G., 1970, Geology of the north end of the Ruby Range, southwestern Montana: U.S. Geological Survey Open-File Report 1436, 187 p.

1976a, Geologic map of the northern part of the Ruby Range, Montana: U.S. Geological Survey Miscellaneous In­ vestigations Map 1-951, scale 1:24,000.

1976b, Paleozoic and Mesozoic stratigraphy of the north­ ern part of the Ruby Range, southwestern Montana: U.S. Geological Survey Bulletin 1405-1, 26 p.

1981, Foreland deformation in the northern part of the Ruby Range of southwestern Montana: Montana Geological Society Field Conference and Symposium Guidebook to Southwest Montana, p. 215-224.

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Prin­ ciples of a resource/reserve classification: U.S. Geological Survey Circular 831, 5 p.

U.S. Geological Survey, 1975, Aeromagnetic map of southwest­ ern Montana and east-central Idaho: U.S. Geological Survey Open-File Report 75-655, scale 1:250,000.

Ruby Mountains Wilderness Study Area A21

Viets, J. G., Clark, J. R., and Campbell, W. L., 1984, A rapid, Wodzicki, Antoni, and Krason, Jan, 1981, National uraniumpartial leach and organic separation for the sensitive deter- resource evaluation, Dillon quadrangle, Montana and Idaho:mination of Ag, Bi, Cd, Co, Mo, Pb, Sb, and Zn in surface U.S. Department of Energy Open-File Report (GJQ-geologic materials by flame atomic absorption: Journal of 007(81), 81 p. Geochemical Exploration, v. 20, p. 355-366.

Webring, M. W., 1981, MINC: A gridded program based on Woollard, G. P., 1979, The new gravity system changes in minimum curvature: U.S. Geological Survey Open-File Re- international gravity base values and anomaly values: Jour- port 81-1224, 12 p. nal of Geophysical Research, v. 44, p. 1352-1366.

A22 Mineral Resources of Wilderness Study Areas Southwestern Montana

APPENDIX

DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical charac­ teristics define a geologic environment in which the existence of resources is unlikely. This broad category embraces areas with dispersed but insignificantly mineralized rock as well as areas with few or no indications of having been mineralized.

MODERATE mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics indicate a geologic environment favorable for resource occurrence, where interpretations of data indicate a reasonable likelihood of resource accumulation, and (or) where an application of mineral-deposit models indicates favorable ground for the specified type(s) of deposits.

HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical charac­ teristics indicate a geologic environment favorable for resource occurrence, where interpretations of data indicate a high degree of likelihood for resource accumulation, where data support mineral-deposit models indicating presence of resources, and where evidence indicates that mineral concentration has taken place. Assignment of high resource potential to an area requires some positive knowledge that mineral-forming processes have been active in at least part of the area.

UNKNOWN mineral resource potential is assigned to areas where information is inadequate to assign low, moderate, or high levels of resource potential.

NO mineral resource potential is a category reserved for a specific type of resource in a well-defined area.

Levels of Certainty

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HJGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

oO.

ocD OLU QC

LL. O

B C

LEVEL OF CERTAINTY

A. Available information is not adequate for determination of the level of mineral resource potential.B. Available information suggests the level of mineral resource potential.C. Available information gives a good indication of the level of mineral resource potential.D. Available information clearly defines the level of mineral resource potential.

Abstracted with minor modifications from:

Taylor, R. B., and Steven, T. A., 1983, Definition of mineral resource potential: Economic Geology,v. 78, no. 6, p. 1268-1270.

Taylor, R. B., Stoneman, R. J., and Marsh, S. P., 1984, An assessment of the mineral resource potentialof the San Isabel National Forest, south-central Colorado: U.S. Geological Survey Bulletin 1638, p.40-42.

Goudarzi, G. H., compiler, 1984. Guide to preparation of mineral survey reports on public lands: U.S.Geological Survey Open-File Report 84-0787, p. 1, 8.

RESOURCE/RESERVE CLASSIFICATION

IDENTIFIED RESOURCES

Demonstrated

Measured IndicatedInferred

UNDISCOVERED RESOURCES

Probability Range (or) Hypothetical i Speculative

ECONOMIC

MARGINALLY

ECONOMIC

SUB-

ECONOMIC

Reserves

Marginal Reserves

Demonstrated Subeconomic Resources

Inferred Reserves

Inferred

Marginal Reserves

InferredSubeconomic

Resources

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from U. S. Bureau of Mines and U. S. Geological Survey, 1980, Principles of a resource/reserve classification for

minerals: U. S. Geological Survey Circular 831, p. 5.

GEOLOGIC TIME CHART Terms and boundary ages used by the U.S. Geological Survey, 1986

EON

Phanerozoic

Proterozoic

Archean

pre- Arc

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean

-hean 2

PERIOD

Quaternary

Neogene

Subperiod

Tertiary

Paleogene

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian Carboniferous

Periods Mississippian

Devonian

Silurian

Ordovician

Cambrian

3800? -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late Early

Late Middle Early

Late Middle

Early

Late Early

Late Middle Early

Late Early

Late Middle Early

Late Middle

Early

Late Middle Early

Late Middle Early

-

BOUNDARY AGE IN

MILLION YEARS

- 0.010

- 1.7

- 5

_ TO

- 55

_ CC

- 96

one

-~ 240

- 290

-~ 330

*3cn

- 410

- 500

- ~ 570 '

- 900

- 1600

- 2500

- 3000

- 3400

4550

Rocks older than 570m.y. also called Precambrian, a time term without specific rank.

Informal time term without specific rank.

U.S. GOVERNMENT PRINTING OFFICE: 1987 773-038/40,035 REGION NO. 8